1. Field of the Invention
The present invention relates to a semiconductor memory device and a method of controlling a semiconductor memory device, and in particular, relates to a semiconductor memory device which has an external connection unit configured to connect with a host device and a main unit which has a built-in semiconductor memory unit that stores data of the host device, and to a method of controlling the semiconductor memory device.
2. Description of Related Art
In recent years, a semiconductor memory device, particularly a semiconductor memory device that stores data in a flash memory being a nonvolatile storage medium, has been developed. Now, such semiconductor memory devices are widely used as auxiliary memory devices for host devices such as personal computers, cellular phones, digital cameras, etc. As such host devices have come to handle data in large capacity, development of flash memories has progressed to achieve flash memories with larger capacity and which are further densified. In this respect, in recent years, NAND type flash memories in particular have come to be used frequently.
In a NAND type flash memory, electric charges injected to a charge accumulation layer through an insulating film are taken as information in a form of digital bit, and information is to be read out by measuring a change in a threshold voltage of a transistor depending on the amount of electric charges.
A USB memory device (hereinafter to be referred to as “USB memory”) can be mentioned as an example of a small size semiconductor memory device, with good portability, that uses a NAND type flash memory. USB memory memories are widely used with personal computers, etc.
For example, Japanese Registered Utility Model No. 3118657 discloses a variable USB memory which performs signal access with a USB transmission interface or a multi-memory card transmission interface that includes transmission interfaces, etc. The transmission interfaces that the multi-memory card transmission interface includes may be an SD transmission interface, an MMC transmission interface and so forth.
As the USB memory has become larger in capacity, a user can store a large number of files in a single USB memory. Data to be stored in the USB memory can be roughly classified into two types of data; one type of data being data that is stored for some time for repeated use, and the other type of data being data that is stored for temporal use and can be deleted without any problem once it is used. Data stored in the USB memory for the purpose of data delivery, for instance, is temporarily used and can be deleted without any problem after use. In such case, however, since there should be plenty of storage capacity left in the USB memory, it often happens that a user will not delete such deletable data, i.e. unnecessary data, from the USB memory. Accordingly, the USB memory will have unnecessary data accumulated therein.
When there are a large number of files stored in the single USB memory, the user may have difficulties in finding out a desired file from the multiple files being stored. Moreover, when a large number of files are being stored in the single USB memory, the user may have difficulties in even determining which files are necessary and which are not. Particularly, it will be difficult to determine whether the file is necessary or not after some time has passed since the file was first stored. Therefore, there are possibilities that the user will not be able to delete unnecessary files or will accidentally delete necessary files. At any rate, it is a burdensome work for the user to find out unnecessary files and delete such files.
On the other hand, USB memories are used quite often in delivering data to/from other persons. In such case, even when there is available storage capacity in the USB memory, storing the data in the USB memory for delivery while the USB memory has other data being stored, and connecting the USB memory to a host device that is used by someone else is questionable, since there is a possibility of data leakage. In other words, in connecting the USB memory to a host device that is used by another person, it is preferable that a USB memory which does not store any data other than the data for delivery is used. However, for this purpose, the user is required to delete all the data stored in the USB memory for storing the data for delivery.